1. Field of the invention
The present invention relates to an inductor and, more particularly, to a small-size inductor capable of being installed in a small devices, such as a wrist watch or a pocket size calculator.
2. Description of the Prior Art
Recently, electronic wrist watches and electronic pocket size calculators and other similar devices employ an inductor for producing a step-up voltage, which is used, for example, for actuating a buzzer to produce a sound. Such an inductor has a very small size, for example, a diameter of about 2-5 millimeters and a height of about 2-4 millimeters, and, therefore, it is referred to as a small-size inductor.
A prior art small-size inductor of the above described type is shown in FIG. 1, and which includes a base member 3 made of ferrite, a terminal plate 9 made of electrically non-conductive material, such as synthetic resin, a coil 11 formed by a very thin lead wire and a cap member 12 also made of ferrite. The base member 3 has a circular bottom wall 3a, a core 10 mounted at the center of the bottom wall 3a, and a cylindrical side wall 3b having a pair of recesses 1 and 2 formed at opposite sides thereof. The terminal plate 9 seats on the bottom wall 3a of the base member 3 such that an opening 8 of the terminal member 9 receives the core 10 and a pair of tongues 6 and 7 of the terminal member 9 extends outwardly from the cylindrical wall 3b through the recesses 1 and 2, respectively. The coil 11 is also mounted on the core 10, and a pair of lead wires extending from the coil 11 are wound around and connected to terminals 4 and 5, respectively, provided at tongues 6 and 7. The cap member 12 includes a cylindrical wall having a pair of recesses formed therein. The cap member 12 is mounted on the base member 3 to define a magnetic circuit path through the center hole and around the outer face of the coil 11.
According to the prior art inductor, the thickness of the terminal plate 9 affects the height of the inductor such that the thicker the terminal plate 9, the greater the height of the inductor. Therefore, in order to reduce the size of the inductor, particularly its height, one approach is to reduce the thickness of the terminal plate 9. When the terminal plate 9 is made considerably thin, however, the tongues 6 and 7 will be easily bent, which results in damage of the small-size inductor. For example, when a part feeder mounts the small-size inductor automatically on a circuit board 13, the vibration of the feeder is transmitted to the terminal plate 9, causing bending of the tongues 6 and 7. When tongues 6 and 7 are bent, they may be broken, or the lead wires connected to the tongues 6 and 7 may be cut.
Another approach to reduce the size of the small-size inductor is to reduce the thickness of the walls defining the base member 3 and the cap member 12. Since the base member 3 and the cap member 12 are made of ferrite, the thickness of such walls can not be made thinner than a certain thickness to ensure a certain level of inductance and also mechanical strength.
Because of the above reasons, the prior art small-size inductor has a disadvantage in that its size can not be made smaler than a certain limited size.
Also, when mounting the prior art inductor on a circuit board 13, it is necessary to form a circular opening 14 in the circuit board 13 so as to receive the bottom wall 3a resulting in contact between electrodes 13a and 13b deposited on the circuit board 13 and the tongues 6 and 7, respectively. If the openings 14 were not provided, the tongues 6 and 7 would be held in a spaced relation with the electrodes 13a and 13b, respectively, causing difficulty in applying solder between the terminal 4 and the electrode 13a and also between the terminal 5 and the electrode 13b.
From the above, it should be understood that the prior art small-size inductor has another disadvantage in that the circuit board 13 for mounting the inductor must be previously provided with opening 14, resulting in increase in the number of manufacturing steps, and at the same time, the back surface of the circuit board 13 where the opening 14 is formed can not be used for mounting or forming circuit parts, resulting in decrease of available areas for mounting or forming circuit parts.
Furthermore, since the prior art small-size inductor has the tongues extending outwardly from the base member 3, the maintenance of the inductors before being mounted on the circuit board requires much care.